Method for reducing dilation balloon cone stiffness

ABSTRACT

A method for stretch blow molding dilatation balloons for angioplasty catheters having a significantly reduced cone thickness without sacrifice in burst strength is achieved by utilizing a mold whose cavity includes arcuate walls defining the balloon&#39;s end cones and a predetermined minimal distance from the side edges of the mold to the points where the arcuate walls intersect with a smaller diameter balloon stem portion. Utilizing this mold and providing for three longitudinal stretching sequences, one prior to, one during and one following radial expansion of the heated plastic parison, results in an improved balloon exhibiting reduced cone stiffness.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to dilatation balloon catheters of thetype employed in percutaneous transluminal angioplasty procedures, andmore particularly to a method of molding such balloons to reduce theircone stiffness and thereby improve the maneuverability in smaller andmore tortious passages of the vascular system.

II. Discussion of the Prior Art

Dilatation balloon catheters are well known for their utility intreating the build-up of plaque and other occlusions in blood vessels.Typically, a catheter is used to carry a dilatation balloon to atreatment site, where fluid under pressure is supplied to the balloon,to expand the balloon against a stenotic lesion.

The dilatation balloon is affixed to an elongated flexible tubularcatheter proximate its distal end region. When the balloon is expanded,its working length, i.e., its medial section, exhibits a diametersubstantially larger than that of the catheter body on which it ismounted. The proximal and distal shafts or stems of the balloon havediameters substantially equal to the diameter of the catheter body.Proximal and distal tapered sections, referred to herein as “cones”,join the medial section to the proximal and distal shafts, respectively.Each cone diverges in the direction toward the medial section. Fusionbonds between the proximal and distal balloon shafts and the catheterform a fluid-tight seal to facilitate dilation of the balloon when afluid under pressure is introduced into it, via an inflation port formedthrough the wall of the catheter and in fluid communication with theinflation lumen of the catheter.

Along with body tissue compatibility, primary attributes considered inthe design and fabrication of dilation balloons are their strength andpliability. A higher hoop strength or burst pressure reduces the risk ofaccidental rupture of the balloon during dilation. Pliability refers toformability into different shapes, rather than elasticity. Inparticular, when delivered by the catheter, the dilatation balloon isevacuated, flattened and generally wrapped circumferentially about thecatheter in its distal region. Thin, pliable dilatation balloon wallsfacilitate a tighter wrap that minimizes the combined diameter of thecatheter and the balloon during delivery. Furthermore, pliable balloonwalls enhance the catheter “trackability” in the distal region, i.e.,the ability of the catheter to bend in conforming to the curvature invascular passages through which it must be routed in reaching aparticular treatment site.

One method of forming strong, pliable dilatation balloons ofpolyethylene terrathalate (PET) is disclosed in U.S. Pat. No. RE. 33,561(Levy). A tubular parison of PET is heated at least to its second ordertransition temperature, then drawn to at least triple its originallength to axially orient the tubing. The axially expanded tubing is thenradially expanded within a heated mold to a diameter about triple theoriginal diameter of the tubing. The form of the mold defines theaforementioned medial section, shafts and cones, and the resultingballoon has a burst pressure greater than 200 psi.

Such balloons generally have a gradient in wall thickness along thecones. In particular, larger dilatation balloons, e.g., 3.0-4.0 mmdiameter (expanded) tend to have a wall thickness in the working lengthin the range of from 0.010 to 0.020 mm. Near the transition of the coneswith the working length or medial section, the cones have approximatelythe same wall thickness. However, the wall thickness diverges in thedirection away from the working length, until the wall thickness nearthe proximal and distal shafts is in the range of 0.025 to 0.040 mm nearthe associated shaft or stem.

The increased wall thickness near the stems does not contribute toballoon hoop strength, which is determined by the wall thickness alongthe balloon medial region. Thicker walls near the stems are found toreduce maneuverability of the balloon and catheter through a tortiouspath. Moreover, the dilatation balloon cannot be as tightly wrappedabout the catheter shaft, meaning its delivery profile is larger andlimiting the capacity of the catheter and balloon for treatingocclusions in smaller blood vessels.

U.S. Pat. No. 4,963,133 (Noddin) discloses an alternative approach toforming a PET dilation balloon, in which a length of PET tubingcomprising the parison is heated locally at opposite ends and subjectedto axial drawing to form two “necked-down” portions, which eventuallybecome the opposite ends of the completed balloon. The necked-downtubing is then simultaneously axially drawn and radially expanded with agas. The degree to which the tubing ends had been necked-down is said toprovide control over the ultimate wall thickness along the wallsdefining the cones. However, it is believed that the use of the Noddinmethod results in balloons exhibiting a comparatively low burstpressure.

Copending application Ser. No. 08/582,371, filed Jan. 11, 1996, U.S.Pat. No. 5,733,301 describes a method for reducing cone stiffness byusing a laser to ablate and remove polymeric material from the coneareas after the balloon is blown. It is preferable that the desiredresult be obtained during the balloon molding operations obviating theneed for additional post molding operations.

Therefore, it is an object of the present invention to provide a methodfor stretch blow molding dilatation balloon having a high burst pressureand hoop strength, but with reduced material mass in the balloon cones,thus reducing cone stiffness and improving the trackability, crossingprofile, stenosis recross and balloon retrieval, via a guiding catheter.

SUMMARY OF THE INVENTION

To achieve these and other objects of the invention, there is provided amethod of making dilatation balloons with reduced cone stiffness. Themethod comprises the steps of first providing a mold having a cavityincluding a cylindrical center segment defining a working length of adilatation balloon body where the center segment is of a predetermineddiameter. The mold cavity also includes two opposed end segments, eachhaving an arcuate cone shape tapering from the predetermined diameter ofthe center segment to a smaller desired balloon shaft diameter. The sideedges of the mold are dimensioned to be within about 0.05 in. of thetermination point of the arcuate cone at the smaller desired balloonshaft diameter.

Next, a tubular polymeric parison of a predetermined diameter and wallthickness is placed with a mold and the parison has the opposed endsthereof extending beyond the side edges of the mold, the opposed endsbeing clamped in a tensioning fixture. The mold is heated to bring thetemperature of the parison near or above the glass transitiontemperature of the polymeric material comprising the parison. Thetensioning fixture is then longitudinally displaced relative to the moldto initially longitudinally stretch the parison by a predeterminedamount to introduce a degree of longitudinal orientation and to neckdown the tubular parison to a lesser diameter.

Following this initial longitudinal stretch, a second longitudinalstretching operation is initiated and as the tensioning fixture is beingmoved to achieve a second stretch, a gas is injected into the tubularparison to radially expand the parison to a limit defined by the moldcavity. At this point, the wall thickness in the working length of theballoon and in its cones is a function of the degree of longitudinal andradial stretching as well as the gas pressure applied to effect theradial expansion.

Following inflation of the balloon within the mold, a third longitudinalstretch is performed by further displacing the tensioning fixturesrelative to the mold. It is the third stretch within the above-describedmold that is found to remove material from the cone area as the tubingis drawn down to a desired size for a catheter shaft. Removal ofmaterial from the cone area renders them more pliable than balloonsprepared in the same way but not subjected to longitudinal stretchingfollowing the radial expansion of the balloon within the mold. The thirdstretch also creates an increased number of nucleation sites forcrystallization to occur.

After the third stretch operation is terminated, the temperature of themold is increased such that the biaxially oriented balloon reaches itscrystallizing temperature for effectively locking the molecularstructure in place.

Following crystallization, the mold is cooled below the glass transitiontemperature of the polymer so that the crystallization structure of theballoon is not lost. Once the mold has sufficiently cooled, it can beopened and the balloon removed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top elevational schematic view of the equipment used incarrying out the method of the present invention;

FIG. 2 is an enlarged view of one of the jaws of the mold showing thedesired profile of the mold cavity used in preparing dilatation balloonshaving reduced cone stiffness;

FIG. 3 is a drawing helpful in understanding the manner in which themold cavity shape is arrived at; and

FIG. 4 is a flow chart of the steps employed in preparing dilatationballoons exhibiting reduced cone stiffness.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated schematically the apparatusfor stretch blow molding dilatation balloons for later assembly on tocatheter body stock in the fabrication of dilatation balloon catheters.The mold itself is indicated generally by numeral 10 and comprises firstand second mold halves 12 and 14 which when abutting one another at aparting line 16 define an internal mold cavity 18. The mold halves orjaws can be opened or spread apart to allow placement of a tubularparison therein. The opposed ends of the parison 22 and 24 are clampedin a tensioning fixture including clamping jaws 26 which are mounted onrails 28 and 30 for longitudinal movement therealong.

As those skilled in the art appreciate, the mold 10 incorporates heatingelements (not shown) and appropriately positioned temperature sensorsfor monitoring the mold temperature and sending temperature informationback to a microprocessor-based controller for maintaining preciseclosed-loop control of the temperature of the mold and of the parisoncontained in it. Likewise, a suitable linear encoder (not shown) isoperatively coupled to the translatable clamping fixtures 26 to providepositional information to the microprocessor-based controller wherebythe degree of longitudinal stretch imparted to the parison 20 can beprecisely controlled.

The equipment for stretch blow molding shown in FIG. 1 also includes ameans for introducing a gas 32, under pressure, into the lumen of thetubular parison 20 and for monitoring and controlling that pressureagain, using closed-loop control.

Except for the mold cavity 18 formed in the mold halves 12 and 14, theequipment used in carrying out the method of the present invention isaltogether conventional. The mold cavity employed is unique, as is theoperation whereby the cone segments of the balloons to be formed in itare made to contain less material than in conventional designs.

FIG. 2 is a view looking at the interior of one of the jaws 12 or 14 andshowing the preferred profile of the mold cavity 18.

The portion of the balloon between the dashed construction lines A-Adefine the working length of a dilatation balloon formed therein andthis portion of the balloon is generally cylindrical. The portion of themold between construction lines A and B form the cones and, as can beseen from FIG. 2, the cones do not have a linear taper. They areslightly arcuate in the zone between the construction lines A and B. Theportion of the mold between the construction lines B and C willultimately comprise the shaft portion of the balloon formed in the moldcavity 18.

The following table sets out typical mold dimensions in stretchblow-molding a dilatation balloon having a working length of 20 mm andan expanded diameter of 4.0 mm. These dimensions are illustrative onlybecause the various dimensions change depending upon the size of theballoon to be formed.

TABLE I Dimension Magnitude (Inches) A-A .763 B-B 1.532 C-C 1.557 B-C.025 A-B .372 R₁ .882 R₂ 1.010

With reference to FIG. 3, for any size balloon diameter, the radiusedballoon ends of the mold are designed using the following graphicalconstruction technique:

1. The horizontal centerline 32 for the mold is first established.

2. Construction lines 34 above and below the horizontal center line 32are established to define the desired balloon diameter.

3. Construction lines 36 above and below the center line establish thedesired balloon shaft diameters for both the proximal and distal ends.

4. The vertical center line 38 for the mold is set.

5. Lines 40 and 40′ define the desired working length of the balloonbody on either side of the vertical center line 38.

6. Construction lines 42 are created at the points of intersections oflines 36 and 40 such that lines 42 form a desired angle with respect toline 36. An angle of 12° is typical. Each of lines 42 should cross thehorizontal center line 32 of the mold. Construction lines 42 determinethe length of the end of the balloon.

7. Construction line 44 is created at the intersection of lines 36 and42. Construction line 44 indicates the boundary for the end of theballoon and the transition to the balloon shaft.

8. Arcs 46 are next constructed. Arc 46 is a three point arc, and itshould pass through the intersection of lines 34 and 40, and lines 42and 44. The end point of the arcs 46 should be chosen so that they aretangent to line 34 at the intersection of lines 34 and 40.

9. Construction lines 42 can now be erased and the portion of the arcs46 to the left (outside) of construction line 44 can also be erased.

10. Displace construction line 44 to the left by 0.025 in. to 0.25 in.establish the left end of the mold which is depicted in FIG. 3 byconstruction line 48.

11. The lines 36 to the right (inside) of construction line 44 and tothe left (outside) of construction line 48 are trimmed to form the shortland of the mold.

12. Construction line 44 can now be erased and lines 34 trimmed to theleft (outside) of line 40 of the left half of the mold.

13. The foregoing construction steps are then repeated for the rightside of the mold to form the other balloon end.

As will be explained in further detail hereinbelow, by providing thearcuate cone segments and the short cylindrical shaft segments(dimension B-C in Table I), it is possible to remove polymeric materialfrom the cone portions of the mold by providing a third stretch to theparison following inflation of the parison to achieve radialorientation.

Using the mold created using the techniques outlined above in theapparatus of FIG. 1, dilatation balloons exhibiting a reduced conethickness as compared to prior art stretch blow molding operations canbe achieved. Referring to FIG. 4, there is illustrated a flow chart ofthe steps used to prepare such improved dilatation balloons. In carryingout the method, a precut length of a suitable tubular parison is placedin the mold so as to span the mold cavity in the longitudinal direction.The opposed ends of the parison are clamped by the tensioning member 26.The mold is partially closed about the tubular parison 20 and a gas at arelatively low pressure is introduced into the lumen of the parison anda slight tension is applied to eliminate sagging of the parison whensubsequently heated.

Following this initial setup and pretensioning, the mold 10 is heated upto a desired temperature which depends upon the thermoplastic materialinvolved. Generally speaking, the mold is heated to a temperature whichis above the glass transition temperature. For PET, the mold maytypically be heated to 175°. Once this temperature is reached, themolding operation can be begin.

The parison is subjected to a first stretching operation to initiatelongitudinal orientation in the plastic. The degree of stretch varieswith the tube size (wall thickness) and the tube material. This firststretch which for a PET parison may be in the range of ¼ in. to{fraction (1 1/2)} in. at each end thereof, not only results in somelongitudinal orientation, but it also necks down the original tubingcomprising the parison to a smaller diameter.

After the prestretch (first stretch), the mold is completely closed anda second longitudinal stretch is initiated. During the time that thesecond stretch is occurring, the balloon is fully inflated by injectingan inert, dry gas, e.g., nitrogen, under relatively high pressure intothe lumen of the parison to thereby radially expand the parison to fillthe mold. The gas pressure depends on tubing thickness and the desiredwall thickness of the resulting balloon but will typically be in therange of from 50 psi to about 400 psi. The wall thickness of theresulting balloon is a function of both the longitudinal stretch and theradial stretch employed. There is also an interaction between thepressure and the degree of longitudinal stretch on the thickness of theresulting balloon wall. Generally speaking, the higher the pressure, theless the wall is thinned by the longitudinal stretching.

With continued reference to the flow chart of FIG. 4, followinginflation of the balloon and while the balloon is still subjected to thepressure of the inflation gas, the parison is longitudinally stretched athird time. Because of the arcuate shape of the mold in the zone thereofdefining the end cones and because of the short dimension B-C (FIG. 2and Table I), the third longitudinal stretch is effective to removematerial from the cone area of the balloon and to simultaneously drawthe tubing down to a desired size thereby providing a thinner shaftportion for later attachment to the catheter body.

Defining the stretch ratio as the ratio of the length after the stretchdivided by the length prior to the stretch, for a PET polymer the firststretch ratio may be in the range of from 1.005 to 2.0, that for thesecond stretch in the range of from 1.05 to 3.0 and for the thirdstretch in the range of from 1.1 to 4.0.

Following the third stretch operation, the temperature of the mold isincreased to the crystallizing temperature of the polymer employed toeffectively “freeze” the molecular structure resulting from thelongitudinal and radial orientation in place. The crystallizing steptakes place with the balloon pressurized to the same inflation pressureearlier applied during the balloon inflation step. This helps to ensurethat the balloon walls in the working area will remain at the samethickness after the third longitudinal stretch and subsequentcrystallizing.

The mold can now be cooled down back below the glass transitiontemperature for the polymer and, following that, the mold can be openedand the clamps released. The portion of the parison outside of the moldis then trimmed off and the balloon is ready to be mounted on a catheterbody.

Comparative tests were run on balloons prepared in accordance with themethod of FIG. 4 when using a mold having a profile like that of FIG. 2with balloons fabricated using a prior art “two stretch” molding processhaving all of the steps of FIG. 4 except the third stretch followingballoon inflation and in a mold that had linear (rather than arcuate)cone profiles. These specific parameters that were compared were derivedby advancing a plurality of dilatation catheters having balloonsmanufactured in accordance with the method of the present invention andballoons manufactured in accordance with the described prior art througha test fixture. The test fixture had a tortuous path and located atdiffering spots within the tortuous path were a Palmez-Schatz stent anda Wallstent® Endoprosthesis. The purpose of this test was to evaluatethe forces required to push the catheter through the fixture and theability of the catheter to pass through each of the stents withoutgetting caught by the stent's structure. The average force that wasrequired to pass the conventional catheter through the test fixture was695.9 grams. This is to be compared with 390.5 grams required to beapplied to the catheters having balloons made in accordance with thepresent invention to traverse the same test fixture. This representsapproximately a 44 percent reduction in tracking force.

A further test was conducted to assess the force required to re-cross astenosis following balloon inflation. Balloons made in accordance withthe method of the present invention in the mold cavity made as describedherein showed an approximate decrease of 18 percent in the stenosisrecross force when compared to balloons molded in the conventional “twostretch” process.

Testing further revealed that the balloons molded with the “threestretch” process of the present invention required the lowest force towithdraw the balloon catheter through a guiding catheter. The force towithdraw the balloons prepared in the three stretch process was about28% less than the force necessary to withdraw balloons made using theprior art two stretch process.

Balloons made in accordance with the three stretch process of thepresent invention were able to be guided through the stent blocks. Theconventional balloons made using the two stretch process were notcapable of being pushed through the stents, even with considerableeffort.

The improved performance of dilatation balloons made in accordance withthe present invention is believed to be due to the extraction ofmaterial from the cone areas of the balloon taking place during thethird stretch. The process of the present invention produces a highdegree of molecular orientation, yielding balloons with high strengthand simultaneously a reduced balloon wall thickness, balloon conethickness and balloon shaft diameter. This eliminates the need forsubsequent balloon processing following the balloon blowing operation.

What is claimed is:
 1. A method of making dilatation balloons withreduced cone stiffness, comprising the steps of: (a) providing a moldhaving a cavity therein including a center section of a predetermineddiameter defining a working length for a balloon to be formed thereinand opposed end cone segments, each defined by an arcuate wall tangentto a wall defining the generally cylindrical center section andterminating in a cylindrical end segment corresponding to a desiredshaft size for the balloon to be formed therein, the mold having opposedside edges spaced less than 0.25 inch from a point of intersection ofthe arcuate wall and the cylindrical end segment; (b) placing a tubularparison of a predetermined polymeric composition across the mold cavity,the tubular parison having opposed ends extending outwardly from theopposed side edges of the mold; (c) clamping the opposed ends of thetubular parison in longitudinally displaceable tensioning fixtures; (d)heating the mold to a temperature above the glass transition temperatureof the polymeric composition of the parison; (e) longitudinallydisplacing the tensioning fixtures relative to the mold a first time toeffect a first predetermined stretch ratio; (f) subsequentlylongitudinally displacing the tensioning fixture relative to the mold asecond time to effect a second predetermined stretch ratio whilesimultaneously injecting a gas, under pressure, into the tubular parisonto radially expand the parison against the walls defining the moldcavity and thereby form a balloon having a generally cylindrical centersegment, a pair of opposed cone segments and a pair of opposed shaftsegments; (g) further longitudinally displacing the tensioning fixturerelative to the mold a third time to effect a third stretch ratio, thespacing of the opposed ends of the mold and the arcuate wallconfiguration of the mold end cone segments configured to effectselective thinning of the balloon cone and shaft segments; (h) heatingthe mold to the crystallizing temperature of the polymeric composition;(i) cooling the mold to a temperature below the glass transitiontemperature of the polymeric composition; and (j) removing the resultingballoon from the mold.
 2. The method as in claim 1 and further includinga step of pretensioning the tubular parison prior to step (d).
 3. Themethod as in claim 1 wherein the polymeric composition comprises PET. 4.The method as in claim 3 wherein the first predetermined stretch ratiois in a range of from 1.005 to 2.0.
 5. The method as in claim 3 whereinthe second predetermined stretch ratio is in a range of from 1.05 to3.0.
 6. The method as in claim 3 wherein the third stretch ratio is in arange of from 1.1 to 4.0.
 7. The method as in claim 1 wherein thetubular parison is a co-extrusion of Nylon 12 over PET.
 8. The method asin claim 1 wherein the gas injected is at a pressure in a range of from50 psi to 400 psi.
 9. A method of fabricating a dilatation balloon in astretch blow molding operation comprising the steps of: (a) providing amold having a cavity formed therein defining a desired shapeconfiguration of a dilatation balloon to be formed therein, the moldincluding a cylindrical central section and opposed generally conicalend sections tapering to a reduced diameter shaft segment, each shaftsegment extending axially to a side edge, wherein the generally conicalend sections comprise arcuate boundaries defining the opposed generallyconical end sections; (b) placing a tubular parison of a polymericcomposition having a predetermined diameter and wall thickness acrossthe mold with opposed ends of the parison extending outward beyond theside edges of the mold; (c) clamping the opposed ends of the parison ina tensioning fixture; (d) heating the mold to a temperature above theglass transition temperature of the polymeric composition; (e)simultaneously inflating and longitudinally displacing the tensioningfixture relative to the mold to thereby stretch the parison to form aballoon within the mold cavity and thereby form a balloon having agenerally cylindrical center segment, a pair of opposed generallyconical end segments and a pair of opposed shaft segments; (f)subjecting the balloon of step (e) to a further longitudinal stretchwithin the heated mold to draw polymeric material from the generallyconical end sections without appreciable thinning of the central sectionthereof, the spacing of the opposed ends of the mold and the arcuateboundary configuration of the general conical end segments configured toeffect selective thinning of the balloon cone and shaft segmentsrelative to the central portion; (g) heating the mold to a crystallizingtemperature of the polymeric composition; (h) cooling the mold back downbelow the glass transition temperature of the polymeric composition; and(i) removing the balloon from the mold.
 10. The method as in claim 9wherein the arcuate boundaries are tangent at one end to a segmentdefining the central section of the balloon and intersect the segmentdefining the balloon shaft at another end.